The present invention relates to a method of producing a film of a nitrogen-doped II-VI group compound semiconductor.
Light sources for use in optical recording and/or reproduction of information are requested to generate light beams of shorter wavelengths for increased recording density and reproducing resolution. Semiconductor light-emitting devices for generating light beams of shorter wavelengths include green and blue laser diodes and light-emitting devices. Of particular interest among those semiconductor light-emitting devices is a semiconductor light-emitting device of a II-VI group compound semiconductor having a large energy band gap which is made of at least one of elements belonging to the group II such as Zn, Mg, Cs, Hg, Be, etc. and at least one of elements belonging to the group VI such as S, Se, Te, etc., e.g., a semiconductor light-emitting device having a light-emitting layer of ZnCdSe.
"Electronic Letters" Vol. 29, No. 16, 1993 discloses the continuous oscillation at room temperature with a wavelength of 523.5 nm of a semiconductor light-emitting device which comprises a GaAs substrate and an SCH (Separate Confinement Heterostructure) disposed therein that is comprised of a ZnNgSSc cladding layer, a ZnSSe guide layer and a ZnCdSe active layer.
"Applied Physics Letters" Vol. 57, No. 20, 1990 discloses the fabrication of p-type ZnSe in which Na--Nd is 1.times.10.sup.17 cm.sup.-3 (where Na represents an acceptor concentration and Nd a donor concentration) by effecting an MBE (Molecular Beam Epitaxy: Molecular Beam Epitaxy) of a ZnSe layer with a beam of free radicals of nitrogen molecules.
"Japanese Journal of Applied Physics Letters" Vol. 30 No. 2A, 1991 shows that when p-type ZnSe was produced by nitrogen molecule radicals according to the above revealed process, the percentage of activated nitrogen atoms in ZnSe was only 0.2%.
One film producing apparatus for producing a film of a p-type II-VI group compound semiconductor by applying a plasma of nitrogen is an MBE (Molecular Beam Epitaxy:: Molecular Beam Epitaxy) apparatus which is schematically shown in FIG. 17, for example. The MBE apparatus, which is of a type of vacuum evaporation apparatus, has a chamber 40 with a vacuum degree (evacuated down to 10.sup.-10 Torr) by an ultra-high vacuum evacuating device (not shown), the vacuum chamber 40 housing a substrate holder 42 for holding a substrate 41 on which a film of a II VI group semiconductor is formed.
The vacuum chamber 40 supports a plurality of molecular beam sources (K cells) of such a II-VI group compound semiconductor which are directed toward the substrate 41. A plasma generation source 44 is also supported on the vacuum chamber 40 for applying a plasma of nitrogen to the substrate 41. The plasma generation source 44 is in the form of an ECR (Electron Cyclotron Resonance) plasma cell as illustrated. The plasma generation source 44 comprises a plasma generation chamber 44R having magnets 45, a microwave terminal 46 disposed in the plasma generation chamber for supplying a microwave, and a nitrogen gas inlet pipe 47 disposed in the plasma generation chamber for supplying a nitrogen gas.
When molecular beams are applied from the molecular beam source 43 to the substrate 41, a film of a II-VI group compound semiconductor, for example, is epitaxially deposited on the substrate 41. For epitaxial growth of a nitrogen-doped p-type II-VI group compound semiconductor on the substrate 41, a nitrogen gas is converted into a plasma due to electron cyclotron resonance by applying a magnetic filed and a microwave in the plasma generation source 44, and the plasma of nitrogen is applied from a plasma outlet 48 of the plasma generation source 44 to the substrate 41. In this manner, a nitrogen-doped II-VI group semiconductor is epitaxially grown on the substrate 41 upon application of the molecular beams from the molecular beam source to the substrate 41. A preliminary chamber 49 is connected to the vacuum chamber 40 for introducing the substrate 41 into and removing the substrate from the vacuum chamber 40.
The plasma of nitrogen contains ionized N.sub.2 ions, N ions, N.sub.2 radicals, N radicals and a large number of electrons. It has been reported that the ionized nitrogen N is not activated in a II-VI group semiconductor and does not contribute to electric conductivity ("Journal of Crystal Growth" vol. 86 (1988), p. 329).
It is highly desirable to increase the reliability and service life of a semiconductor light-emitting device, e.g., a semiconductor laser or a light-emitting diodes, of a II-VI group semiconductor described above, e.g., ZnMgSSe, ZnSSe or ZnCdSe. To this end, it is important to lower the defect density and increase the crystallinity.
However, when a plasma of a p-type impurity of nitrogen is applied to dope a II-VI group semiconductor while a film thereof is being deposited, this compound semiconductor has no sufficient crystallinity, which is a bottleneck in increasing the reliability and life time thereof.
As a result of research activities, the inventors have found that when a plasma of nitrogen is applied to dope a II VI group compound semiconductor while a film thereof is being deposited, the application of electrons produced by the generation of the plasma greatly affects the creation of crystal defects. The inventors have also revealed that the existence of N.sub.2 ions and N ions which do not contribute to the electric conductivity of a II-VI group compound semiconductor, i.e., which are not activated, is likely to induce crystal defects, and high-speed neutral particles excited by the generation of the plasma cause damage (damage) to the crystal upon collision with a deposited semiconductor film. For example, the spectrum of light emitted at 77.degree. K from a p-type ZnSe crystal that is deposited on a GaAs substrate and doped with nitrogen by a plasma of nitrogen includes a light emission, referred to as a Y-line, due to a crystal defect, as well as a band-end emission which indicates nitrogen incorporated as an acceptor, as shown in FIG. 18. However, the spectrum of light emitted from a ZnSe crystal that is grown simply in nitrogen includes no Y-line as shown in FIG. 19. It can therefore be seen that the Y-line is produced by a plasma of nitrogen.